Object detection method and system

ABSTRACT

A method and system for monitoring a region of interest are presented. Incident radiation is transmitted towards the region of interest with a certain transmitting angle and with a predetermined angular intensity distribution of the incident radiation. The transmitting angle defines a plane of propagation of the incident radiation, the region of interest being located within this plane. Reflections of the incident radiation are collected with a solid angle of collection intersecting with said plane. A region of intersection presents a detecting window of a predetermined geometry containing at least a portion of the region of interest. The collected radiation coming from within the detecting window is detected, and output signals indicative thereof are generated.

FIELD OF THE INVENTION

[0001] This invention is generally in the field of proximity sensingtechniques, and relates to a method and system for monitoring a regionof interest to detect an object therein.

BACKGROUND OF THE INVENTION

[0002] Proximity sensing systems aimed at object detection in generalare well known per se and are found in wide use in commercial, policeand military applications. Generally, such systems fall into two broadcategories of systems: “active” systems based on signal transmissiontowards an object and detection of signals returned (reflected) from theobject, and “passive” systems that do not utilize energy transmissiontowards a target to be detected (e.g., infrared or heat-seekingsystems).

[0003] U.S. Pat. No. 4,185,192 discloses a passive optical systemutilizing two detectors oriented such that their optical axes and coneshaped fields of view intersect, thereby creating an overlapping regionbetween these fields of view. By this, only those signals from thedetectors, which are received simultaneously, being thereby indicativeof that the detected light comes from the overlapping region, will causethe generation of a switching signal.

[0004] Another example of such a passive optical system for determiningthe presence of an object by utilizing the creation of a scene at theintersection of two optical paths associated with two detectors isdisclosed in U.S. Pat. No. 4,317,992.

[0005] U.S. Pat. No. 4,396,945 discloses the technique that can beutilized in either an active or passive system for determining theposition and orientation of an object in space. This technique is basedon the determination of the intersection of a line with a plane or withanother line.

[0006] Active systems are also disclosed in U.S. Pat. Nos. 4,590,410 and4,724,480. According to the technique of U.S. '410, which is aimed atdetecting small objects, multiple light emitters and multiple lightdetectors are utilized. According to the technique of U.S. '480, asystem is composed of at least one projector generating a non-planarlight and at least one camera, which are oriented such that the opticalaxes of the camera(s) and projector(s) intersect. By this, the projectorassociated with a first object and the region of intersection associatedwith a second object can be aligned.

SUMMARY OF THE INVENTION

[0007] There is a need in the art to facilitate the detection of anobject in a region of interest by providing a novel method and systemfor monitoring a region of interest.

[0008] The main idea of the present invention consists of defining adetecting window of a known location containing a region of interest tobe monitored so as to detect objects located solely within the detectingwindow, and prevent monitoring of regions outside the detecting window.This is implemented by transmitting incident radiation towards theregion of interest with a certain transmitting angle so as to define aplane of propagation of the incident radiation, and collecting radiationwith a solid angle of collection intersecting with this plane. Thedetecting window is thus a plane-like region of intersection between theangle of collection and the plane of transmission. In order to optimallyuse the energy and maximize the signal-to-noise ratio (SNR) in detectedradiation, the incident radiation is transmitted towards the region ofinterest with a certain predetermined angular intensity distribution.

[0009] It should be understood that the term “monitoring” used hereinsignifies observing or sensing the region of interest either to simplydetect the appearance or existence of an object within the detectingwindow (region of interest), or to enable imaging of the region ofinterest. For example, a detector may be of that kind producinggenerated data indicative of the detected reflections in the form of anindication signal (alarm) indicative of the fact that an object existsin the detecting window. To enable the imaging of an object locatedwithin the detecting window, an appropriate detector (i.e., having asensing surface in the form of one- or two-dimensional array of pixels)and an image processing technique should be used.

[0010] There is thus provided, according to one aspect of the presentinvention, a method for monitoring a region of interest, the methodcomprising the steps of:

[0011] (a) transmitting incident radiation towards the region ofinterest with a certain transmitting angle to define a plane ofpropagation of the incident radiation, and with a predetermined angularintensity distribution of the incident radiation, the region of interestbeing located within said plane;

[0012] (b) collecting reflections of the incident radiation with atleast one solid angle of collection intersecting with said plane, aregion of intersection being a detecting window of a predeterminedgeometry containing at least a portion of said region of interest;

[0013] (c) detecting the collected radiation coming from within saiddetecting window and generating output signals indicative thereof.

[0014] Preferably, to provide the predetermined angular intensitydistribution of the incident radiation, a transmitter unit comprises aspecifically designed beam-shaping element.

[0015] Preferably, in order to increase the quality of detection, thesensitivity distribution of a receiver unit within the sensing surfaceof a detector is adjusted so as to provide substantially uniform outputof the receiver unit for collected radiation components coming fromdifferent locations within the detecting window. Additionally, thesensing surface of the receiver unit can be divided into a plurality ofspatially separated sensing regions, each for collecting a correspondingone of the solid angle segments of the collection angle of the receiverunit.

[0016] According to another aspect of the present invention, there isprovided a system for monitoring a region of interest, the systemcomprising:

[0017] (One) a transmitter unit operable to transmit incident radiationwith a certain transmitting angle defining a plane of propagation of theincident radiation, and with a predetermined angular intensitydistribution of the incident radiation, said region of interest beinglocated within said plane; and

[0018] (Two) at least one receiver unit oriented and operable to collectreflections of the incident radiation with at least one certain solidangle of collection intersecting with said plane, a region ofintersection being a detecting window of a predetermined geometrycontaining at least a portion of said region of interest, to detect thecollected radiation coming from within said detecting window, andgenerate data indicative thereof.

[0019] Preferably, in order to prevent stray light (particularly, solarradiation) from reaching the receiver unit, and prevent the reflectionof the incident radiation from the ground, the receiver and transmitterunits are oriented such that the field of view of the receiver, whilebeing directed towards the detecting window, extends downwards from thehorizon, and the incident radiation, while propagating towards thedetecting window, is directed upwards from the horizon. For the samepurpose, namely, to prevent the stray light from being detected, anadditional receiver unit can be used to collect the reflections of theincident radiation coming from within the detecting window, but with adifferent collection angle. For example, the two receivers are orientedsuch that their collection angles are symmetrical with respect to thedetecting window plane. This arrangement prevents the direct solarradiation from being detected simultaneously by the two receiver units.

[0020] Generally, more than one receiver units may be provided beingassociated with the same transmitter unit and being operable forreceiving radiation components propagating with a corresponding one ofsolid angle segments of said solid angle of collection. More than onetransmitter units may be provided associated with a corresponding numberof the receiver units, thereby monitoring the corresponding number ofportions (detecting windows) of the region of interest.

[0021] The transmitter unit comprises a radiation source (preferably alaser diode) generating the incident radiation of a predeterminedspectral range, a collimator, and a beam-shaping element providing thedesired angular intensity distribution of the incident radiation. Thebeam-shaping element is of a refractive type, and is composed of one ormore refractive blocks. Each of the refractive blocks has a first activesurface facing the radiation source, a second active surface and anactive medium enclosed between the first and second surfaces, and isformed by an array of facets. The orientation of a surface region of thefirst active surface defined by the facet with respect to the secondactive surface and a length of this surface region are defined by theangular intensity distribution of the incident radiation to be createdby the radiation propagation through this specific facet.

[0022] Thus, according to yet another aspect of the present invention,there is provided a beam-shaping element for use in a transmitter unitfor transmitting radiation with a predetermined angular intensitydistribution, wherein

[0023] the beam-shaping element comprises at least one refractive blockhaving a first active surface for facing a radiation source of thetransmitter unit, a second active surface and an active medium enclosedtherebetween;

[0024] the first active surface of said at least one refractive block isformed by an array of facets, orientation of a surface region of thefirst active surface defined by each of the facets with respect to thesecond active surface and a length of said surface region being definedby the predetermined angular intensity distribution, I(θ), of thetransmitted radiation to be produced by radiation propagation throughsaid at least one refractive block, θ being a steering angle created bythe facet of the refractive block.

[0025] Preferably, the beam-shaping element is also designed so as to bevery robust to linear intensity variation of the emitted radiation atthe entrance of the beam-shaping element (at the first active surface).To this end, the array of facets is composed of two sets, which aresymmetrically identical with respect to the central axis of therefractive block.

[0026] The receiver unit comprises a spectral filter preventing thedetection of radiation outside said predetermined frequency spectralrange, a radiation collecting assembly and a detector. A sensing surfaceof the detector has a predetermined geometry selected in accordance withthe geometry of the detecting window (region of interest). The detectoris implemented with a specifically constructed variable sensitivityfilter, such that the output of the detector is substantially uniformfor the collected reflections coming from different locations in thedetecting window. This filter also reduces the overall stray light(background radiation) and back-scattered radiation from particles thatmay occasionally exist in the detecting window and scatter light towardsthe receiver unit.

[0027] Thus, according to yet another aspect of the present invention,there is provided a detector for use in a system for monitoring a regionof interest, the detector comprising a radiation collecting assembly,and a sensing surface for receiving collected radiation and generatingoutput representative thereof, wherein the sensing surface has adesirably variable sensitivity distribution such that the output signalscorresponding to the collected radiation components coming fromdifferent locations within said region of interest are substantiallyequal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to understand the invention and to see how it may becarried out in practice, a preferred embodiment will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

[0029]FIG. 1 is a schematic illustration of the main components of asystem according to the invention;

[0030]FIG. 2 illustrates the construction of a transmitter unit of thesystem of FIG. 1;

[0031]FIGS. 3a to 3 c more specifically illustrate the features of abeam-shaping element of the transmitter unit of FIG. 2;

[0032]FIG. 4 illustrates the construction of a receiver unit of thesystem of FIG. 1; and

[0033]FIG. 5 more specifically illustrates the geometry of a sensingsurface of the receiver unit given in an enlarged scale.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Referring to FIG. 1, there is illustrated a system 10 accordingto the invention for monitoring a region of interest 12, namely, fordetecting the appearance or existence of an object within this region(detecting window). The system 10 comprises such main constructionalparts as a transmitter unit 14 for generating incident radiation (light)and transmitting it towards the region of interest 12 (detectingwindow), and a receiver 16 unit for receiving reflections of theincident radiation coming from the region of interest 12 and generatingdata indicative thereof.

[0035] The transmitter unit 14 generates an incident light beam L_(inc)of a predetermined spectral range and transmits it with a certain acuteangle of propagation θ and certain angular intensity distribution of theincident light. The incident light beam has a substantially plane shape(the so-called “sheet of light”), and the region of interest 12 islocated in this plane P. The receiver unit 16 collects light L_(col) (ofthe predetermined spectral range) with a solid angle of collection φ.

[0036] The detecting window 12 presents a region of intersection betweenthe plane P (defined by the transmitter unit 14) and the solid angle oflight collection φ (defined by the receiver unit 16). In other words,the detecting window 12 is a region cut by the solid angle of lightcollection φ from the plane P. In the present example, the solid angleof collection φ has a rectangular cross-section, thereby defining arectangular shape of the detecting window 12.

[0037] It should be understood that what the receiver unit 16 actuallyreceives are reflections of the incident light produced within a regiondefined by the detecting window 12 (i.e., light reflected from an objectlocated within the window 12). The system 10 is thus capable ofmonitoring the detecting window containing the region of interest, i.e.,detecting, and possibly also imaging, objects located within thedetecting window 12, while not detecting any object located outside thiswindow.

[0038] With regard to the preferred relative position of the transmitterand receiver units, the following should be noted. The receiver unit 16is oriented such that its field of view, while being directed towardsthe detecting window 12, extends downwards from the horizon. Thetransmitter unit 14 is oriented such that the incident radiation, whilepropagating towards the detecting window 12, is directed upwards fromthe horizon.

[0039] The above positioning of the transmitter and receiver units 14and 16 is associated with the following. On the one hand, when straylight (in particular, the solar radiation) directly reaches the sensingsurface of a detector, it causes the saturation of the detector. On theother hand, the reflections of incident laser radiation from the groundcan reach the sensing surface of a detector, thereby causing a falsealarm thereof. By locating the transmitter and receiver units in theabove-described manner, the solar radiation is prevented from reachingthe detector (irrespective of the current location of the Sun), and theincident radiation is prevented from reaching the ground.

[0040] It should be noted, although not specifically shown, that, inorder to prevent the stray light from being detected, the system maycomprise an additional receiver unit constructed similar to the receiverunit 16, but oriented to collect the reflections of the incidentradiation coming from within the detecting window 12 symmetrical to thatof the unit 16 with respect to the detecting window plane. Thisarrangement prevents the direct solar radiation from being detectedsimultaneously by the two receiver units.

[0041] Generally, the system may comprise more than one receiver unitassociated with the same transmitter and operable together to collectthe reflections of the incident radiation from the detecting window withthe solid angle of light collection φ. In this case, each of thereceiver units collects a light component propagating with acorresponding one of the angular segments of the entire solid angle oflight collection φ.

[0042] It should also be noted that, in order to monitor the entireregion of interest, the system may comprise more than one transmitterunit, each being associated with one or more receiver unit. In thiscase, each transmitter-receiver arrangement is associated with acorresponding one of the detecting windows that cover together theentire region of interest. In other words, the entire region of interestcan be monitored by dividing it into a number of detecting windows, anddetecting the reflections coming therefrom by the corresponding numberof the transmitter-receiver arrangements.

[0043]FIG. 2 illustrates the main constructional elements of thetransmitter unit 14. The transmitter unit 14 is composed of a laserdiode (radiation source) 18 operating with the predetermined spectralrange, an aspheric collimating lens 20, and a beam-shaping element 22. Alight beam B₁ emitted by the laser diode 18 is collimated by the lens20, and the passage of collimated light B₂ through the element 22produces the incident sheet of light L_(inc). It should be understoodthat the light propagation is shown here schematically, in order tosimplify illustration.

[0044] For the purposes of the present invention, the beam-shapingelement 22 is of a refractive type, having two active surfaces 22 a and22 b enclosing an active medium M with the refraction index ntherebetween. The element 22 is formed of one or more blocks, five suchblocks, generally at 23, being shown in the present example. By usingtwo active surfaces, the maximum angle of propagation of the incidentradiation produced by the beam-shaping element can be increased. Thebeam-shaping element 22 is designed using a specific algorithm, so as toprovide desired angular intensity distribution of the incident light.Additionally, the beam-shaping element is designed so as to be veryrobust to laser intensity variations. As for the aspherical collimatinglens 20, its design is optimized for actual laser junction to achievemaximum collimation capability.

[0045] Turning now to FIGS. 3a-3 c, there are illustrated the mainprinciples underlying the design of the beam-shaping element 22. In FIG.3a, one block 23 of the element 22 is exemplified. The block 23 iscomposed of 2N facets formed by two sets of facets (point-likelocations) F and F′, each set consisting of N facets, and the sets beingidentically symmetrical with respect to the central axis CA of the block23. Thus, the set F is composed of N facets F_(i) (where i=1, . . . ,N), and the identically symmetrical set F′ is composed of N facetsF′_(i) (where i=N+1, . . . , 2N).

[0046]FIG. 3b illustrates the light propagation through one of thefacets F_(i). The emitted collimated light beam B₂ impinges onto thesurface region 22 a defined by the facet F_(i) at an angle φ_(i),successively passes through the medium M and the interface defined bythe second active surface 22 b of the facet F_(i), and ensues from thefacet F_(i) as the light component L^((i)) _(inc) of the output incidentlight L_(inc) with the specific angle of propagation θ_(i) (i.e., theangular segment of the acute angle of propagation θ of the incidentlight). The projection d_(i) of the surface 22 a onto the plane of theother active surface 22 b that actually defines the length of the facetF_(i), is determined in accordance with the light intensity I(θ) whichshould be produced by the light propagation through this facet.

[0047] Considering the entire block 23 composed of 2N facets F_(i), eachangle φ_(i) will have its corresponding angle θ_(i), and, accordingly,the angular intensity distribution I(θ) of the incident light is formedby the intensities of the 2N light components passed through the 2Nfacets of the block 23, i.e., I(θ_(i)).

[0048]FIG. 3c illustrates the angular intensity distribution as producedby the entire block 23 formed by symmetrically-identical facet-setsF_(i) and F′_(i).

[0049] Thus, in order to provide certain desired angular intensitydistribution of the incident light, the facets of the block of thebeam-shaping element should be designed accordingly. The algorithmunderlying the above design of the block of the beam-shaping element 22consists of the following. The desired output angular intensitydistribution of block 23, I(θ), is quantized into a discrete set ofangles θ_(i), each angle defining the tangential of the first activesurface 22 a which can be found by solving the following transcendentalequation:

θ_(i)=arc sin[n sin(φ_(i)−arc sin( ))] (1)

[0050] wherein θ_(i) is the specific angle of propagation of lightensuing from the facet for which the tangential φ_(i) must be found.

[0051] Taking into account that the length d_(i) of the correspondingfacet F_(i) for each angle θ_(i) is proportional to the relative outputintensity at that angle I(θ_(i)), each facet F_(i) of the block 23 iscalculated. By this, the desired angular intensity distribution of lightensuing from the beam-shaping element can be obtained.

[0052] In order to make the angular intensity distribution of thetransmitter unit substantially independent of the linear non-uniformityof the laser diode radiation, the two sets F and F′ of successive facets(i.e., F₁, F₂, . . . , F_(N), and F_(N+1), F_(N+2), . . . , F_(2N)) areidentically symmetrical with respect to the central axis CA of the block23. This is implemented by rotating each facet in one set the angle of180° with respect to its corresponding facet in the other set (i.e., F₁and F_(2N), F₂ and F_(2N−1), . . . , F_(N) and F_(N+1)).

[0053] The above technique dramatically increases the robustness of theelement 22 to the possible linear intensity variation in the laser 18,thereby providing substantially non-sensitivity of the incident lightproduced by the transmitter unit to any linear intensity variation oflight emitted by the laser at the entrance to the beam-shaping element(i.e., at the surface 22 a).

[0054] It should be noted that in order to further increase therobustness of the beam-shaping element to intensity variations, theentire arrangement of 2N facets (block 23) can be scaled andperiodically repeated. The scaling factor and number of periods solelydepends on the quality of the laser 18: the less the uniformity of lightdistribution emitted by the laser, the lower scale and greater number ofperiods should be used.

[0055]FIG. 4 illustrates the main components of the receiver unit 16,which is of a pseudo-imaging kind. The receiver unit 16 comprises anoptical system 30 (constituting a radiation collecting assembly), adetector 32, and a protective window 34 accommodated in front of theoptical system 30 (with respect to the direction of propagation of lightimpinging onto the receiver unit), all being accommodated in a metalhousing 36. The optical system 30 is composed of a lens assembly 38 anda laser wavelength matched spectral filter 40 accommodated in front ofthe lens assembly 38. The lens assembly 38 includes a first asphericallens 38 a and a second spherical lens 38 b (with respect to a directionof propagation of the collected radiation through the receiver unit),which are mounted adjacent to each other and define a common opticalaxis OA of light propagation within the receiver unit towards a sensingsurface 32 a of the detector 32.

[0056] The receiver unit 16, having the above design of the opticalsystem 30, is capable of providing a very large field of view and depthof focus, to meet the requirements of the imaging system. The opticalsystem 30 is capable of creating an image of the detecting window (12 inFIG. 1) on the sensing surface 32 a. Angular imaging resolution may varyover the field of view in order to provide the uniform spatialresolution of the detecting window.

[0057] The sensing surface 32 a of the detector 32 is specificallydesigned so as to provide a desired sensitivity distribution within thesensing surface 32 a of the detector resulting in substantially uniformoutput from different points on the sensing surface. This is associatedwith the fact that the design of the detecting window (i.e., itsorientation with respect to the optical axis OA) may result in that thedetected light components coming from locations (points) in thedetecting window differently distanced from the plane of the sensingsurface 32 a, will have different intensities. Hence, to provide uniformoutput signals from the detector 32, a specifically constructed variablesensitivity filter should be implemented in the detector 32.

[0058] As shown in FIG. 5, illustrating the sensing surface 32 a in anenlarged scale, the shape of the sensing surface 32 a is selected inaccordance with the shape of the detecting window 12 by projecting thecontours of the detecting window 12 onto the sensing surface 32 athrough the optical system 30 of the receiver unit 16. In the presentexample of the rectangular-shaped detecting window 12, the projection ofthe points P₁-P₄ results in points P_(1′)-P_(4′), forming the trapezoidshape of the sensing surface 32 a.

[0059] It should be noted, although not specifically shown that, inorder to increase the depth of focus, the detector 32 should be orientedsuch that its sensing surface 32 a is not perpendicular to the opticalaxis OA of the optical system 30, but rather appropriately inclinedthereto. The angle of inclination can be determined by any suitabletechnique, for example, by the least square method, which is known perse.

[0060] To enable the light detection with substantially uniform outputof the detector, the inventors have developed an iteraction algorithmfor the calculation of the sensitivity filter function to providedesirably variable sensitivity of the detector. This algorithm consistsof the following:

[0061] It is known that, in the first approximation, the transmissionfunction of the filter T₁(x,y) can be calculated based on the followingformula: $\begin{matrix}{{T_{1}\left( {x,y} \right)} = {c \cdot \frac{R_{1} \cdot R_{2}^{2}}{I\left( {\theta \left( {\overset{\rightarrow}{R}}_{1} \right)} \right)}}} & (2)\end{matrix}$

[0062] wherein x and y are the coordinates in the filter plane, i.e.,the sensing surface 32 a; {right arrow over (R₁)} is a vector connectingthe transmitter unit 14 with a specific point in the detecting window12; I(θ({right arrow over (R)}₁)) is the intensity of a signal generatedby the transmitter unit 14 and transmitted in the {square root over(R₁)} direction; {square root over (R₂)} is a vector connecting thespecific point in the detecting window 12 with the receiver unit 16; andc is a normalization factor chosen to achieve maximum overalltransparency.

[0063] The above equation (2), however, does not take into account theactual point spread function (PSF) of the system, which should be takeninto account when dealing with the performance of a real system. In thiscase (i.e., considering the PSF), the intensity distribution I₁ of thedetector output reads: $\begin{matrix}{{I_{1}\left( {x_{d},y_{d}} \right)} = {\underset{\underset{\underset{plane}{detector}}{{over}\quad {the}}}{\int\int}{T_{1}\left( {x,y} \right)}{P\left( {{x - x_{d}},{y - y_{d}}} \right)}{x}{y}}} & (3)\end{matrix}$

[0064] wherein P(x−x_(d), y−y_(d)) is the actual PSF of the optics ofthe receiver unit, (x_(d), y_(d)) being the coordinates of a currentlocation in the detector plane (sensing surface 32 a).

[0065] Thus, for each point in the detecting window 12, thecorresponding PSF on the detector plane 32 a can be found by using theknown available ray-tracing code. In the present example, the OSLO SIXsoftware product commercially available from Sinclair Optics Inc., USAwas used. Other known codes, such as Zemax or Code V, can be used. Then,this PSF is multiplied by T₁(x,y) and numerically integrated over theentire detector plane, thereby producing an actual signal on thedetector (equation (3) above).

[0066] This process is repeated for an appropriate grid of points in thedetecting window 12, thereby producing the sensitivity map on thedetector plane 32 a. If PSF were an ideal impulse response, namely deltafunction, the map of sensitivity would be uniform. In practice, however,owing to the fact that the actual PSF is blurred by real optical system,a uniform sensitivity map cannot be obtained.

[0067] In order to achieve the uniform output of the detector, thedesirably variable sensitivity map of the detector should be provided.To this end, the iteration algorithm is based on correcting (varying)the transmission function T₁(x,y) by normalizing it with respect to thecalculated sensitivity map, thereby producing a new transmissionfunction T₂(x,y): $\begin{matrix}{{T_{2}\left( {x_{d},y_{d}} \right)} = \frac{T_{1}\left( {x_{d},y_{d}} \right)}{I_{1}\left( {x_{d},y_{d}} \right)}} & (4)\end{matrix}$

[0068] This process is repeated until appropriate uniformity of thedetector output is obtained. More specifically, for k-th level ofiteration of the transmission function T_(k)(x,y) (i.e., k-th step ofthe optimization procedure), the corresponding value of the intensityfunction I_(k)(x,y) is calculated as described above (equation (3)),and, accordingly, the next iteration level T_(k+1)(x,y) in the iterationprocedure is found. The iteration procedure continues until the requireduniformity of the detector output I(x,y) is achieved.

[0069] The simulations have shown that the iteration procedure convergesvery fast (about 5 iterations), producing quite uniform output from thedetector (better than 5% P-V).

[0070] The above variable transmission (sensitivity) of the detector canbe implemented by appropriately patterning the sensing surface 32 a ofthe detector in different ways. For example, the pattern may befabricated in a separate, specifically coated glass plate and placed onthe sensing surface. Alternatively, the correspondent transmissionfilter (pattern) or the different sensitivity can be implementeddirectly on the detector by means of ion implantation of the detector'ssensing surface (e.g., Si—B).

[0071] Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the preferred embodiment ofthe invention as hereinbefore exemplified without departing from itsscope defined in and by the appended claims.

1. A method for monitoring a region of interest, the method comprising:(i) transmitting incident radiation towards the region of interest witha certain transmitting angle to define a plane of propagation of theincident radiation, and with a predetermined angular intensitydistribution of the incident radiation, the region of interest beinglocated within said plane; (ii) collecting reflections of the incidentradiation with a solid angle of collection intersecting with said plane,a region of intersection being a detecting window of a predeterminedgeometry containing at least a portion of said region of interest; (iii)detecting the collected radiation coming from within said detectingwindow and generating output signals indicative thereof.
 2. The methodaccording to claim 1, wherein said transmitting of the incidentradiation provides propagation of the incident radiation upwards fromthe horizon, and said collecting is carried out with a field of viewextending downwards from the horizon.
 3. The method according to claim1, wherein the transmission of the incident radiation with thepredetermined angular intensity distribution comprises the step of:passing radiation emitted by a radiation source through a beam-shapingelement comprising at least one refractive block having a first activesurface facing the radiation source, a second active surface, and anactive medium enclosed therebetween, the first active surface of said atleast one refractive block being formed by an array of facets,orientation of a surface region of the first active surface defined byeach of the facets with respect to the second active surface and alength of said surface region being defined by the predetermined angularintensity distribution, I(θ), to be produced by radiation propagationthrough said at least one refractive block, θ being a steering anglecreated by the facet of the refractive block.
 4. The method according toclaim 3, wherein the orientation of the surface region of the firstactive surface defined by each of the facets with respect to the secondactive surface and the length of said surface region are determined asfollows: the predetermined angular intensity distribution, I(θ), isquantized into a discrete set of angles θ_(i), each defining atangential φ_(i) of said surface region by solving a followingtranscendental equation:$\theta_{i} = {\arcsin \left\lbrack {n\quad {\sin \left( {\phi_{i} - {\arcsin \left( \frac{\sin \quad \phi_{i}}{n} \right)}} \right)}} \right\rbrack}$

 wherein θ_(i) is a specific angle of propagation of radiation ensuingfrom the i^(th) facet; each of the facets is calculated taking intoaccount that a projection of said surface region of each facet onto thesecond active surface is proportional to a relative output intensity atthe corresponding angle.
 5. The method according to claim 3, whereinsaid array of facets of the refractive block is composed of two sets,which are symmetrically-identical with respect to a central axis of therefractive block.
 6. The method according to claim 1, wherein thedetection of the reflections comprises: providing desirably variablesensitivity distribution within a sensing surface of a detector, therebyproviding substantially equal output signals of the detectorirrespective of locations within the detecting window where thereflections are produced.
 7. The method according to claim 6, whereinthe desirably variable sensitivity distribution is provided by formingthe sensing surface with a pattern providing a desired sensitivity mapwithin the sensing surface.
 8. The method according to claim 7, whereinsaid pattern is determined by performing an iteraction algorithm forcalculating a sensitivity filter function to be such as to providesubstantially equal values of the output signals corresponding to thereflections of the incident radiation coming from different locations inthe detecting window.
 9. The method according to claim 8, wherein saiditeraction algorithm consists of determining, for each location in thedetecting window, a corresponding value of PSF on the sensing surface;multiplying the determined PSF value by a transmission function T₁(x,y)of the filter, and numerically integrating it over the entire sensingsurface; repeating the same for an appropriate grid of locations in thedetecting window; and correcting the transmission function T₁(x,y) bynormalizing it with respect to the calculated sensitivity map.
 10. Themethod according to claim 1, wherein the collection of the reflectionsof the incident radiation comprises collection of components of thereflections of the incident radiation propagating with angular segmentsof said solid angle of collection.
 11. The method according to claim 1,wherein the reflections of the incident radiation are collected with anadditional solid angle of collection.
 12. The method according to claim1, wherein the two solid angles of collection are symmetricallyidentical with respect to the plane of the detecting window.
 13. Themethod according to claim 9, and also comprising transmitting theincident radiation towards the region of interest with at least oneadditional transmitting angle and with a predetermined angular intensitydistribution of the incident radiation, the additional transmittingangle intersection with the additional solid angle of collection in saidplane, thereby defining at least two detecting windows locating in thesame plane and containing at least two portions of the region ofinterest, respectively.
 14. A system for monitoring a region ofinterest, the system comprising: (a) a transmitter unit operable totransmit incident radiation with a certain transmitting angle defining aplane of propagation of the incident radiation and with a predeterminedangular intensity distribution of the incident radiation, said region ofinterest being located within said plane; and (b) at least one receiverunit oriented and operable to collect reflections of the incidentradiation with a certain solid angle of collection intersecting withsaid plane, a region of intersection being a detecting window of apredetermined geometry containing at least a portion of said region ofinterest, to detect the collected radiation coming from within saiddetecting window, and generate data indicative thereof.
 15. The systemaccording to claim 14, and also comprising at least one additionalreceiver unit.
 16. The system according to claim 15, wherein said atleast one additional receiver unit collects the reflections of theincident radiation with a solid angle of collection symmetricallyidentical to said certain solid angle of collection with respect to saidplane.
 17. The system according to claim 14, wherein the transmitterunit is oriented such that the incident radiation propagates towards thedetecting window upwards from the horizon, and the receiver unit isoriented such that its field of view extends downwards from the horizon.18. The system according to claim 14, wherein the transmitter unitcomprises a radiation source, a collimator, and a beam-shaping element.19. The system according to claim 14, wherein said beam-shaping elementis of a refractive type comprising at least one refractive block havinga first active surface facing the radiation source, a second activesurface, and an active medium with a certain refraction index enclosedtherebetween, the first active surface of said at least one refractiveblock being formed by an array of facets, orientation of a surfaceregion of the first active surface defined by each of the facets withrespect to the second active surface and a length of said surface regionbeing defined by the predetermined angular intensity distribution, I(θ),to be produced by radiation propagation through said at least onerefractive block, θ being a steering angle created by the facet of therefractive block.
 20. The system according to claim 19, wherein saidarray of facets of the refractive block is composed of two sets, whichare symmetrically identical with respect to a central axis of therefractive block.
 21. The system according to claim 19, wherein saidbeam-shaping element comprises at least one additional refractive block,scale and number of the refractive blocks depending on the distributionof radiation emitted by the radiation source within the first activesurface of the beam-shaping element.
 22. The system according to claim14, wherein the receiver unit comprises a spectral filter, a radiationcollecting assembly, and a detector, which has a sensing surface withpredetermined geometry and desirably variable sensitivity distribution.23. The system according to claim 14, wherein the sensing surface isdivided into regions, each region receiving a corresponding one of solidangle segments of the solid angle of collection.
 24. The systemaccording to claim 22, wherein the sensing surface is shaped andpatterned in accordance with the geometry of the detecting window andits orientation with respect to an optical axis of propagation of thecollected light.
 25. The system according to claim 22, wherein thegeometry of the sensing surface is defined by a projection of thedetecting window onto the sensing surface through said radiationcollecting assembly.
 26. The system according to claim 22, wherein thesensing surface is made of silicone with a pattern formed by ionimplantation of boron thereby providing desired distribution of atransmission function of the sensing surface.
 27. The system accordingto claim 22, wherein the sensing surface is covered with a maskproviding desired distribution of a transmission function of the sensingsurface.
 28. A beam-shaping element for use in a transmitter unit fortransmitting radiation with a predetermined angular intensitydistribution, wherein the beam-shaping element comprises at least onerefractive block having a first active surface for facing a radiationsource of the transmitter unit, a second active surface, and an activemedium enclosed therebetween; the first active surface of said at leastone refractive block is formed by an array of facets, orientation of asurface region of the first active surface defined by each of the facetswith respect to the second active surface and a length of said surfaceregion being defined by the predetermined angular intensitydistribution, I(θ), to be produced by radiation propagation through saidat least one refractive block, θ being a steering angle created by thefacet of the refractive block.
 29. A detector for use in a system formonitoring a region of interest, the detector comprising radiationcollecting assembly, and a sensing surface for receiving collectedradiation and generating output representative thereof, wherein thesensing surface has a desirably variable sensitivity distribution suchthat the output signals corresponding to the collected radiationcomponents coming from different locations within said region ofinterest are substantially equal.